The present invention relates to a piston compressor for compressing gas according to the preamble of claim 1.
Such a horizontal piston compressor is known from, for example, EP-A-0 434 607. Piston compressors of the type mentioned in the preamble are generally very large double-acting compressors with several cylinders and are used in particular in the oil and petrochemicals industry. The forces of inertia which are the result of the large mass of the reciprocating parts of the compressor are a major reason for placing the cylinders horizontally in the frame. Although a large part of these forces can be compensated for by balancing the movements of the piston/piston rod units, the remaining forces on the frame of the compressor can be absorbed more readily by the bedplate of the compressor if they are directed horizontally instead of vertically.
However, the horizontal position of the cylinder(s) in the case of the type of piston compressor described in the preamble leads to the generally known problem with regard to supporting the reciprocating piston/piston rod unit relative to the stationary part of the compressor, i.e. the frame and the cylinder(s) forming part thereof. In the case of the horizontal compressors of this type known hitherto a piston/piston rod unit is supported at the crosshead side by the crosshead which is guided in the frame, and at the other side the piston rests on the bottom part of the wall of the cylinder. In the case of this type of compressor the piston is usually provided with one or more replaceable belts, which lie around the piston in the peripheral direction and project beyond the body of the piston, as described in EP-A-0 434 607. These belts are known as rider rings. Rider rings which do not extend all the way around the piston, but extend only along a bottom segment of the periphery of the piston, are also known.
As described in EP-A-0 434 607, wear of the rider rings leads to run-out, which is permissible only within certain limits. Until now, oil has generally been used as the lubrication between the piston and the cylinder wall in order to prevent excessive wear of the bearing surfaces and the occurrence of run-out. However, in recent years there has been an increasing need for compressors of the "oil-free type", in other words, compressors in which the compressed gas is not polluted by lubricating oil which provides the lubrication between the rider rings around the piston and the cylinder. In order to be able to make a compressor of the type mentioned in the preamble an "oil-free compressor", great attention is paid to the composition of the material of the rider rings and the fastening thereof to the piston. For example, it is known to make the rider rings from materials with advantageous lubricating and wear properties, such as PTFE.
As described earlier, horizontal piston compressors of the type mentioned in the preamble are used mainly in situations where continuous operation is required. The mechanical construction of this type of piston compressor was developed over many decades in such a way that such compressors could operate continuously at high efficiency for years. However, it was found that, despite recent developments, the wear on the rider rings can still be undesirably fast, inter alia, due to parameters which cannot be influenced. This means that in practice the compressors have to be shut down after a few months, in order to measure the wear on the rider rings, and in order to be able to replace any rider rings which may be worn. This maintenance adversely affects the overall efficiency and serviceability of this type of compressor.